Sudden shutdown

[Generalsarge]

My computer will sometimes just shutdown instantly at a random time. The period of time between each time it does it ranges from many weeks to days. I have to turn it on and off quickly,switch on and off on wall to get it to work. Otherwise if i just turn it on after having one of the crashes it makes a series of long and short beeps with black screen. could somone please if they can help me with what is the problem and the solution if possible. Thanks

[Dilbert]

First of all, try a few of these.
http://www.computerhope.com/cgi-bin/yabb/YaBB.cgi?num=1149804407

If this doesn't fix it, let us know and we'll continue further. We also will need to know the following:
Amount of memory
What operating system you use
If you have a brand computer (i.e. Dell, Compaq), please give the make and model, i.e. "Dell Dimension E521" or something like that.
Any recent changes you or someone else has/have made to the system.